Many biologically-active materials, such as vaccines, microbial cultures, etc., are dehydrated for purposes of storage. Methods used in the prior art include freeze-drying and air-drying methods such as spray-drying. Dehydration generally lowers the viability of the materials. Freeze-drying allows higher viability levels than air-drying but it requires long processing times and is expensive. It also causes some level of loss of viability in the dried materials.
It is also known in the art to dehydrate biological and other materials using a resonance chamber type of microwave vacuum dehydrator. This directs microwave energy into a vacuum chamber that serves as a resonance cavity for microwaves. However, particularly where the quantity of material being dried is relatively small, which is commonly the case with biomaterials, controlling the temperature of the material can be difficult. When microwaves are reflected within a resonance chamber, as the material dries the microwave energy output of the apparatus must be absorbed by less and less water and material in the sample. The mass of the material to be processed also has to be matched with the microwave power of the apparatus; quantities of material that are small relative to the microwave power of the apparatus may reach high temperatures when drying because of the abundance of microwave energy absorbed by the material.